1. Field of the Invention
The invention relates to liquid crystal displays (LCDs). More specifically, the invention describes a method and apparatus for reducing electromagnetic interference in a liquid crystal display.
2. Discussion of Related Art
Electromagnetic interference (EMI) is a measure of the amount of interference that an electronic device (the unintentional transmitter) disturbs an intentional receiver. Not surprisingly, EMI is a major concern in design of devices, such as PCs, flat panel monitors, etc, that rely on high speed components, because it determines whether a system, PC motherboard, graphics controller, etc gets approved for sale by the US Department of Commerce. This situation is especially true in designs that feature high speed (e.g., Pentium-class) processors, high-speed buses, and several clock outputs. Typically, EMI testing occurs late in the design process, so failing the test can mean expensive redesign and increased time to market. In addition to material costs, using shielding as a way to reduce EMI significantly increases production complications, further driving up system cost.
There are, however, various techniques to reduce and/or eliminate EMI. One such technique is referred to as pulse shaping which requires control of the output waveshape in order to control higher frequency harmonics. However, pulse shaping does not control the spectral energy of the fundamental but only changes the shape of the rising edge by rounding off the corners and reducing some of the higher frequency components and their energy. Therefore, pulse shaping works if one can control the portion of the waveform near the switching threshold.
An additional problem with pulse shaping is that the balancing act between too much rounding and not enough rounding to achieve the desired EMI reduction is made even more complex since temperature and voltage variations disrupt the balance. This balancing act is further complicated by the fact that techniques used for optimum rounding may not give consistent results from run to run in manufacturing. For example, carefully set capacitive or resistive shaping values change from production lot to production lot requiring overdesign of the system to ensure that process variations leave sufficient EMI control and rise time.
Yet another approach to reducing EMI referred to as slew-rate control manages the rising-edge slope by maintaining an output drive that doesn't overcharge load capacitance. Slew-rate control achieves this maintenance by creating a current-controlled output that avoids having a fast, high current and should theoretically be effective. However, as with pulse shaping, a major issue is maintaining control on a manufacturing lot-to-lot basis and across various voltage and temperature ranges. The design must account for the worst-case process and for both high and low temperatures and voltages. These potential variations are both critical and unpredictable. As a result, slew-rate control is difficult to implement and unreliable.
Finally, the most popular approach to reducing EMI, referred to as spread spectrum technology (SST), spreads the energy of a fundamental frequency to minimize any peaking of energy at specific frequencies. This technique reduces both the fundamental-frequency EMI and the higher frequency harmonic components, significantly reducing overall system EMI radiation without compromising clock-edge rise and fall times (see FIGS. 1A–1B). With lower spectrum-peak amplitudes, a system meets and has more margin for EMI. Spread spectrum is the simplest, most efficient technique and offers the most immunity to manufacturing-process variations. Accordingly, the use of SST has pervaded the motherboard market to the point where it is being used in virtually all designs using chipsets that support a 100 MHz front side bus (FSB) as well as for PCI, CPU, and memory buses. All motherboard chipset vendors are designing their parts to work with spread-spectrum timing signals.
A useful component in the frequency conversion of discrete signals is a direct digital synthesizer (DDS). The DDS usually performs a frequency step-down function. A summation unit adds an n-bit value SF stored in a SF register to the n-bit value from the output of a phase accumulator. The sum is synchronously updated upon each rising edge of a clock signal SCLK. The phase accumulator feeds the n-bit DDS frequency FDDS to the output module, and feeds back FDDS to a summation unit, thereby generating, over some number of SCLK cycles, a staircase periodic signal with a frequency given by the formula in Equation (2) below:                               F          DDS                =                              SF                          2              n                                ⁢                      F            SCLK                                              (        2        )            where FSCLK is the frequency value of SCLK. An output module converts a DDS frequency signal FDDS to a destination clock DCLK. An output module could, for example, convert the staircase waveform into a binary clock signal with frequency FDDS. It should be noted that the jitter in the period of staircase periodic signal is equal to the SCLK period. If the SCLK period varies over a wide range (i.e., has high jitter), then it may be difficult (or impossible) to design the output module to reduce the jitter effectively.
Therefore what is desired is an efficient method and apparatus for reducing EMI using spread spectrum technology by providing a selectable frequency modulated clock signal.